U.S. Caribbean

Freshwater

Freshwater is critical to life throughout the Caribbean. Increasing global carbon emissions are projected to reduce average rainfall in this region by the end of the century, constraining freshwater availability, while extreme rainfall events, which can increase freshwater flooding impacts, are expected to increase in intensity. Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater in coastal aquifers. Increasing variability in rainfall events and increasing temperatures will likely alter the distribution of ecological life zones and exacerbate existing problems in water management, planning, and infrastructure capacity.

Linkage Between Climate Change and Regional Risks

Freshwater availability is a function of rainfall, temperature, evapotranspiration (evaporation and transpiration from plants), land cover, watershed characteristics, water use and management, and water quality, and is dependent on the intensity, duration, frequency, and distribution of rainfall within the island. Availability is also affected by seasonal and annual variability in rainfall as well as long-term climate trends. Climate change will likely result in water shortages (due to an overall decrease in annual rainfall), a reduction in ecosystem services, and increased risks for agriculture, human health, wildlife, and socioeconomic development in the U.S. Caribbean.

Rainfall in the U.S. Caribbean is highly variable across space and time, complicating analyses of trends.39 However, past occurrences of drought or excessive rainfall provide insights into vulnerabilities that may be indicative of the future. Droughts and extreme rainfall events in recent years have resulted in economic loss and social disruption. The most recent drought of 2014–2016 in Puerto Rico and the USVI resulted in severe losses to the agriculture sector, implementation of water rationing by the Puerto Rico Aqueduct and Sewer Authority, drying of wetlands, and reduced habitat quality for freshwater biota, including threatened and endangered species such as the Antillean manatee.40

Freshwater resources are primarily surface waters. In the USVI, desalination plants provide some of the public water supply. In Puerto Rico, management and sustainable use of water resources and infrastructure have been problematic for decades, particularly in terms of storage, distribution, and quality of the public water supply.41,42 In 2013, 57.4% of all water produced was lost in distribution.42 Recurring droughts and sedimentation-induced reductions in reservoir storage present a challenge to freshwater availability.43 One of the principal sources of potable water for Puerto Rico, Loíza reservoir, has lost nearly 40% of its original storage capacity due to sedimentation.44,45

Future Climate Change Relevant to Regional Risks

The greatest risk to freshwater resources may be reduced availability due to drying trends.46 Large uncertainty remains in terms of projected rainfall intensity, duration, and frequency. However, hydrologic model simulations indicate that major reservoirs in Puerto Rico could enter permanent supply deficit as early as 2025 under a higher emissions scenario (SRES A2) (see the Scenario Products section of App. 3) and by 2040 under a lower emissions scenario (SRES B1; Figure 20.7).46

Figure 20.7: Projected Change in Annual Streamflow

Figure 20.7: This figure shows ten-year moving averages of projected annual streamflow leaving Lago La Plata and Lago Loíza. Projections were developed using an estimation of water supply entering the reservoirs and an estimation of withdrawals. The former was developed using a range of global climate models (GCMs; shading indicates averages from all GCMs used in the study) and the mean of that range (gray line). The latter was developed using a conservative population growth rate. Annual streamflow is modeled under a higher emissions scenario (SRES A2; left panel) and a lower emissions scenario (SRES B1; right panel). The solid black line is the historical streamflow through 2012.46 It is important to note these are the best estimates available for projected streamflow and use the older generation of GCMs, which project more drying for the region.28 Source: adapted from Van Beusekom et al. 2016.46

Studies indicate that some locations within the Caribbean may experience longer dry seasons and shorter, but wetter, wet seasons in the future.2,3,4,5,6,8 Extended dry seasons are projected to increase fire likelihood9,10 and affect plant phenology (the timing of important biological events), as well as wildlife dependent on fruiting and flowering.47 Excessive rainfall coupled with poor construction practices, unpaved roads, and steep slopes, which are typical of the Caribbean islands, can exacerbate erosion rates and reduce reservoir capacity, water quality, and nearshore habitat quality.

Rainfall also drives the distribution of ecological life zones in the U.S. Caribbean.48 Projected decreases in rainfall foreshadow relative increases in dry life zones and the shrinkage and disappearance of wetter life zones. Ecological implications of these shifts include changes in biodiversity, carbon cycling, forest composition and structure, and nutrient and water cycling.7 Vulnerable life zones include the unique rainforest habitats in the Luquillo Mountains of Puerto Rico (Figure 20.8).8,49,50 Montane species are shifting their ranges upslope and may reach upper elevational limits as temperatures continue to climb.51 Studies find that cloud levels in the dry season are consistently as low as, or lower than, in the wet season in the Luquillo Mountains, indicating that the cloud forest ecosystem may be more vulnerable to wet-season drought periods than previously assumed.10

Cloud Forests Are Vulnerable to Climate Change

Figure 20.8: Tropical montane cloud forests in the Luquillo Mountains of Puerto Rico are characterized by the …

Challenges, Opportunities, and Success Stories for Reducing Risk

Climate change projections provide new impetus to establish practices that reduce current risks to drought and excessive rain and, by inference, reduce future risks to new conditions. The United Nations Environment Programme has promoted rainwater harvesting in Caribbean Small Island Developing States (SIDS).56,57 The Puerto Rico Technical Scientific Drought Committee also recommended the use of cisterns and other structural measures to capture rainwater in residential areas of the territory, encouraged their use on existing homes, and recommended making them mandatory for new projects.40 These systems not only serve as sources for drinking water but also help in storm water management.58,59,60 Citizens of the USVI are required by law to be directly responsible for their own domestic water supply. The majority of USVI’s residents depend on cistern water and use the public source only when they run out of their cistern water.57

Application of new technologies is vital if losses from water supply distribution systems are to be reduced. Public freshwater supplies are jeopardized by reservoir sedimentation, which can also be harmful to downstream ecosystems as sedimentation rates are reduced downstream. Improving sediment management practices, such as those identified from prior experiences,61 can help sustain reservoir capacities and minimize environmental impacts.

Emerging Issues

Managing freshwater and balancing water use among sectors are emerging as two of the most important issues to the U.S. Caribbean islands. Increasing agricultural production will improve food security and the economy but will be challenging, as water availability is likely to decrease over much of the Caribbean.62 Options for improving water-use efficiency in the agricultural sector include optimizing the management of water infrastructure, applying scientific methods for scheduling irrigation, determining crop water requirements for local crops, using crop suitability modeling to evaluate potential responses to climate change and extreme weather scenarios, plant-breeding for extreme conditions, and implementing methods to improve soil fertility, reduce erosion, and increase carbon storage (Ch. 27: Hawai‘i & Pacific Islands, KM 1).62,63